The present invention relates to fiber amplifiers having filter means for attenuating or removing certain specified wavelengths, and to resonant ring fiber filters for use in such amplifiers.
Doped optical fiber amplifiers consist of a gain fiber the core of which contains a dopant such as rare earth ions. The gain fiber receives an optical signal of wavelength xcexS and a pump signal of wavelength xcexP which are combined by means such as one or more couplers located at one or both ends of the gain fiber. The spectral gain of a fiber amplifier is not uniform through the entire emission band. For example, an erbium doped gain fiber, the gain band of which coincides with the 1550 nm telecommunications window of silica based optical fiber, has an irregular gain spectrum that includes a narrow peak around 1536 nm. Fiber amplifier gain spectrum modification has been employed in fiber amplifiers for such purposes as gain flattening and gain narrowing.
It is known that a gain fiber can include a distributed filter for improving the efficiency of a fiber amplifer and/or tailoring the spectral output thereof. Such a distributed filter/gain fiber has an active ion-doped core that is located along the fiber axis, and it further includes a second, off-axis core that extends parallel to the active ion-doped core. The two cores have different characteristics such as core diameters and/or refractive index profiles. The structure can support at least two core modes, and the propagation constants of the two core modes can be manipulated independently by proper selection of the aforementioned characteristics. The cores can therefore be designed such that their propagation constants are equal at a certain resonant wavelength, xcex0. At wavelength xcex0 the fundamental mode of the structure changes from one core to another. Strong power transfer between the two cores can happen only at a narrow band of wavelengths centered about the resonant wavelength. If the second core contains a light absorbing material, it will absorb at least a portion of the light centered about wavelength xcex0 to provide a filtering function that modifies the fiber amplifier gain spectrum.
It is difficult to make a fiber having two parallel cores because of its lack of circular symmetry. Also, a filter having two parallel cores is polarization dependent.
These disadvantages could be avoided by providing the amplifier with a known coaxial coupler of the type wherein a ring core is concentric with and radially spaced from the central active core to form a device referred to herein as a resonant ring fiber (RRF). At least two modes exist in a RRF. Any mode with most of its power in the core is defined as a core mode, and any mode with most of its power in the ring is defined as a ring mode. The propagation constants of one of the core modes and one of the ring modes of a RRF can be manipulated independently by varying the parameters of the core and ring. The two modes of the RRF structure behave in the same way as the two modes in the parallel core fiber coupler/filter described above, but the RRF is much easier to make using vapor deposition-based conventional fiber fabrication technology; moreover, it is intrinsically not polarization dependent due to its circular symmetry.
In the aforementioned parallel core coupler/filter, differing amounts of power can be attenuated in the off-axis core, depending upon the concentration of light absorbing dopant material contained in that off-axis core. After the parallel core filter is made, the amount of attenuation per unit length therein is fixed. If manufacturing tolerances were such that a predetermined length of fiber did not provide the desired attenuation, it would be desirable to be able to tune the attenuation to the desired value.
An object of the invention is to improve the efficiency of a fiber amplifier and/or tailor the spectral output of a fiber amplifier. Another object is to provide an improved fiber optic filter. Yet another object is to provide a distributed fiber optic filer, the peak filter wavelength and peak attenuation of which can be readily adjusted. Another object is to provide a temperature stable fiber optic filter.
The present invention relates to a distributed filter formed of an optical fiber having a central core, a ring core having an inner radius rR concentric with the central core, an inner cladding region of refractive index ni between the central and ring cores, and a cladding layer of refractive index nc surrounding the ring core. The maximum refractive indices n1 and n2 of the central and ring cores are greater than nc and ni. At least a portion of the optical fiber is subjected to a continuous curvature as by winding it into a coil. The propagation constants of one core mode and one ring mode are different at wavelengths except for wavelength xcex0, whereby a narrow band of wavelengths including xcex0 is coupled between the central core and the ring core and is at least partially radiated, whereby the narrow band of wavelengths is attenuated.
This technology is especially useful for implementation of distributed loss filters in gain fibers utilized in certain fiber amplifier and laser designs where an appropriate ring core is used in addition to the conventional active ion-doped central core to obtain spectral gain shaping. The ring structure can be designed to provide the appropriate loss for a certain fiber coil size at those wavelengths where the fiber amplifier exhibits amplified spontaneous emission.
The peak attenuation wavelength xcex0 of a fiber of given outside diameter can be measured, and it may be determined that a fiber having a different value of rR (and thus outside diameter) will result in the correct value of wavelength xcex0. Thereafter, the draw blank can be drawn to a fiber having an outside diameter different from the given outside diameter. It may be beneficial to add more cladding material to the original draw blank or to etch some cladding material from the original draw blank prior to drawing the modified resonant ring fiber; these steps could result in a fiber having a different value of rR and yet retain the given outside diameter. Also, a drawn fiber can be stretched to decrease its outer diameter.
The cladding portions of the fiber can consist of a base glass such as SiO2 and the central and ring cores can comprise SiO2 doped with different amounts of a refractive index increasing dopant such as GeO2 or Al2O3 to increase the refractive index. The filter can be athermalized by employing an appropriate co-dopant such as B2O3 together with the index raising dopant to balance out the thermal dependence of the propagation constants of the central and ring cores.